The invention relates to large-scale electric power transmission networks, and, more particularly, to a method for assessing the stability of an electric power transmission network, a power network stability estimating device and a computer program product according to the preamble of claims 1, 10 and 11, respectively.
Electric power transmission and distribution systems or networks comprise high-voltage tie lines for connecting geographically separated regions, and substations for transforming voltages and for switching connections between lines. Power generation and load flow in a network with several substations is controlled by a central energy management system. An important issue in the control of a power generation and load flow is to keep the network stable, i.e. to avoid voltage collapses and swings. A method for assessing network stability, based on voltage margins, is described in the paper xe2x80x9cUse of local measurements to Estimate Voltage-Stability Marginxe2x80x9d, K. Vu et al., Power Industry Computer Applications (PICA) May 12-16, 1997, IEEE, and in xe2x80x9cVoltage instability predictor (VIP) and its applicationsxe2x80x9d, K. Vu et al., Power Systems Computation Conference (PSCC) June 1999. Both articles are herewith incorporated by reference. These articles describe a xe2x80x9cVoltage Instability Predictorxe2x80x9d (VIP) which measures currents and voltages locally in order to infer a proximity to voltage collapse. The concept of the VIP is shown in FIG. 1. One part of an electric power system is treated as a power source, another part as a load. The power source is represented by its Thxc3xa9venin equivalent 1 with a source voltage Et and a Thxc3xa9venin or source impedance Zt. The load is represented by an apparent load impedance Za. Both the Thxc3xa9venin impedance Zt and the apparent load impedance Za are estimated from the current and voltage measurements by a VIP device 2. The relation of these impedances, expressed by a stability margin or power margin, indicates how close the power system or network is to collapsing.
Given a first measurement point comprising a first voltage phasor V1 and a first current phasor I1, a first load impedance Za1 is computed as   Za1  =            V1      I1        .  
Given a second measurement point comprising a second voltage phasor V2 and a second current phasor 12, the Thxc3xa9venin impedance Zt is computed as   Zt  =      -          xe2x80x83        ⁢                            V2          -          V1                          I2          -          I1                    .      
Note that the phasors as well as the impedances are vector quantities. A practical difficulty lies in estimating the Thxc3xa9venin impedance Zt correctly. In particular, if the apparent load impedance Za corresponding to the first and second measurement points does not change, then the value for the Thxc3xa9venin impedance Zt is invalid.
In U.S. Pat. No. 5,631,569, a system for temporary measurement of network parameters is shown. The magnitude of the Thxc3xa9venin impedance Zt is estimated by taking advantage of natural fluctuations in the power network, which cause measured voltages and currents to change. A RMS (root mean square) voltage and an RMS current are measured cyclically. For each cycle, a scalar apparent load impedance is computed. The Thxc3xa9venin impedance is only computed if two cycles with disparate load impedances are identified. In the preferred embodiment, a 10% difference in load impedance is considered useful for calculating Thxc3xa9venin impedance. Successive values of the Thxc3xa9venin impedance are stored and statistical data is generated and maintained for the successive values. When a standard deviation falls below one sigma, the mean of the values is displayed.
The VIP however, requires not just a single value for occasional display, but a continuous generation of estimated values for the Thxc3xa9venin impedance Zt that are suited for the generation of a stability margin and for use in a network or substation control system.
It is therefore an object of the invention to create a method for assessing the stability of an electric power transmission network, a power network stability estimating device and a computer program product that provide a continuous generation of a stability margin that is suited for use in a network or substation control system.
These objects are achieved by a method for assessing the stability of an electric power transmission network and a power network stability estimating device according to the claims 1 and 10, respectively, and by a computer program product according to claim 11.
In the inventive method and device for assessing the stability of an electric power transmission network, where at least one pair of measurements comprising a first and a second measurement point, each measurement point comprising a voltage and a current phasor, is processed and where a Thxc3xa9venin impedance and a stability margin value are computed, a validity indicator is computed which depends on whether there exists a predetermined difference between the first and second measurement points and, if this is the case, also on whether there exists a predetermined difference between corresponding first and second load impedances. From all validity indicators associated with all of the at least one pair of measurements a quality indicator is computed that is associated with the Thxc3xa9venin impedance value and with the stability margin value.
The invention allows to continuously compute and output a stability margin value and to provide a measure of its quality. This allows an operator to take informed control actions. In a preferred embodiment of the invention, the stability margin value and associated quality indicator are transmitted to a network or substation control system. This allows, for example, the control system to automatically react to a problem in the network, indicated by the stability margin, only when the associated quality indicator qs show that the stability margin is of a high quality, i.e. higher than a predetermined threshold value.
In a preferred variant of the invention, the validity indicator v indicates whether the associated pair of measurements provides a valid, an unchanged or an invalid Thxc3xa9venin impedance.
In a preferred embodiment of the invention, the device comprises means for displaying the quality indicator q and/or means for transmitting the quality indicator q and the stability margin value to a control system that controls at least part of the power network.
A computer program product according to the invention comprises a computer readable medium, having thereon: computer program code means to make, when said program is loaded, the computer execute a method for assessing the stability of an electric power transmission network according to inventive method.
Further preferred embodiments are evident from the dependent patent claims.